Modern oil field operations demand a great quantity of information relating to the parameters and conditions encountered downhole. Such information typically includes characteristics of the earth formations traversed by the borehole, and data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as “logging,” can be performed by several methods including wireline logging, “logging while drilling” (LWD), drillpipe conveyed logging, and coil tubing conveyed logging.
In wireline logging, a probe or “sonde” is lowered into the borehole after some or all of the well has been drilled. The sonde hangs at the end of a long cable or “wireline” that provides mechanical support to the sonde and also provides an electrical connection between the sonde and electrical equipment located at the surface of the well. In accordance with existing logging techniques, various parameters of the earth's formations are measured and correlated with the position of the sonde in the borehole as the sonde is pulled uphole.
In LWD, the drilling assembly includes sensing instruments that measure various parameters as the formation is being penetrated. While LWD techniques allow more contemporaneous formation measurements, drilling operations create an environment that is generally hostile to electronic instrumentation and sensor operations.
In drillpipe or coil tubing conveyed logging, sensing instruments are mounted on a tubing string, which moves the instrument package through an existing borehole. The tubing string enables logging of horizontal well bores without requiring the sensing instruments to tolerate the hostile drilling environment. Typically, the measurement data is stored in internal memory and recovered along with the instrument package.
One example of a logging tool is a wireline dielectric tool. Dielectric tools determine the dielectric constant and conductivity of downhole formations from the real and imaginary parts of the complex propagation constant of electromagnetic waves traveling through the formations. (See, e.g., T. J. Calvert, R. N. Rau and L. E. Wells, “Electromagnetic propagation . . . A new dimension in logging,” presented at the Annual Meeting SPE, Bakersfield, Calif., April 1977, Paper 6542; D. S. Daev, Vysokochastonye Electromagnitnye Melody Issledevity. Skhvazhin:publ.House “Nedra,” Moscow, 1970; and R. A. Meador and P. T. Cox, “Dielectric constant logging, a salinity independent estimation of formation water volume,” presented at the Annual Meeting SPE, Dallas, Tex., Oct. 1, 1975, Paper 5504). By measuring the phase difference and amplitude ratio between two points in the formation, the tool determines the formation resistivity and dielectric constant. These measurements are useful for finding water-filled porosity (Poley, J. Ph., Nooteboom, J. J., de Waal, P. J.: “Use of VHF Dielectric Measurements for Borehole Formation Analysis,” The Log Analyst vo. 19, pp 8-30 May-June 1978), and water saturation can be computed if formation porosity is known. If multiple water saturation measurements are available (e.g., from different types of logging tools), it becomes possible to measure characteristics of the flushed zone.
Existing tool designs have undesirable limitations on accuracy and operating range.
While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to be limiting, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the scope of the appended claims.